as a mark of his affection for the three girls, he left them a
thousand pounds a-piece.

Austen is being sardonic. As a "mark of his affection" £1000 or $90,000 for each is less a mark than a stain. The girls will have to depend on that money for living expenses (it will yield 4% per year or £40) and for a dowry. They attended him through "goodness of heart."

The subtext is the financial dependency of women upon men. The old Gentleman's enthrallment with an ill-bred toddler who charms out of equity in the sense also of fairness comes at the expense of four women whose futures are instantly beggared.